A certain type of motorcycle is equipped with an oil tank device and a conventional oil tank device is provided with an oil tank disposed within vicinity of the head pipe of the body frame.
Engine oil is stored within such an oil tank and fed to respective portions constituting an engine unit so as to lubricate the same. The oil tank is constituted by means of the respective internal cavities of a front rail, a down tube and a bridge tube connecting the tank rail and the down tube and by means of a space defined between two reinforcements secured to both sides of the front tank rail, the down tube and the bridge tube.
An oil passage joint is welded to the lower end portion of the down tube and a rear tank rail is also welded to the lower portion of the front tank rail through means of an oil stopper joint. With the oil tank device described above, however, the difference between the wall thickness of the down tube and the oil passage joint is remarkably large, for example, and since those portions at which the wall thickness difference exists between the down tube and the oil passing joint is not gradually changed when the lower tubes or the like members are welded to the oil passage joint, stress may be concentrated at the welded portion of the oil passage joint.